The present invention relates generally to structural joints in composite structures and, more particularly, to a uniquely configured composite joint that is specifically adapted to minimize transfer of heat across the composite joint such as between an exterior of the composite structure and an interior thereof. The unique arrangement of the composite joint also provides for an effective sealing of the interior of the composite structure such as may be required in a fuel tank of an aircraft.
In many vehicles such as aircraft, the control of heat flow through certain structural areas is of critical importance in the design of the aircraft. More specifically, certain aircraft constructed of composite materials must be adapted to limit the flow of heat from an exterior of the aircraft to an interior thereof. Likewise, it is sometimes desirable to limit the flow of heat across a composite joint out of an interior portion of a composite structure. As is well known in the art, composite structures are typically comprised of composite panels which are constructed as inner and outer face sheet within which a core panel is sandwiched therebetween.
The composite face sheets may be fabricated of any fiber reinforced resin matrix such as fiberglass or graphite epoxy. Core panel materials are typically foam or honeycomb construction. It is also well known that many aircraft integrate fuel tanks into the wing structure. The ability to control or prevent the flow of excess heat into the fuel tanks is important for the overall integrity, performance and safety of the aircraft. Such heat may be generated due to aerodynamic friction as well as due to radiation as may be generated by the sunlight on the aircraft's wing surfaces.
Heat generated by aerodynamic friction on outer surfaces of joint areas of the aircraft may be somewhat mitigated by the use of countersunk fasteners. More specifically, it is common in the industry to employ mechanical fasteners having countersunk heads in joint areas to minimize the amount of aerodynamic drag that would otherwise be collected by protruding fastener heads. However, because most mechanical fasteners include a shank which extends from the exterior to the interior of the aircraft structure, the mechanical fastener provides a path by which thermal heat may readily pass into the interior.
The control of heat flow through non-joint areas of the composite structures may be more easily controlled. For example, thermal conductivity from the outer face sheet toward the inner face sheet may be controlled by reducing the radiative or conductive heat transfer through the core panel. For core panels made of closed cell foams, conductive heat transfer is made difficult due to the large number of small cells that the heat must cross in traveling from one face sheet to another face sheet, such as from an outer face sheet to an inner face sheet. For honeycomb core panels, both radiative and conductive heat transfer between the outer face sheet and the inner face sheet is much easier as most honeycomb cores include hollow cells which provide a direct path for heat flow. Moreover, cell walls of honeycomb core panels provide a direct conductive heat path from the outer face sheet to the inner face sheet.
Control of heat flow in joint areas is more difficult due to the heat path provided by mechanical fasteners. In such joint areas, heat conduction from the outer surface to the inner surface is a function of the cross sectional area of the shank of the mechanical fasteners. Unfortunately, because most mechanical fasteners are typically formed of highly conductive materials such as metallic material, heat is easily conducted through the shank. One attempt to minimize the amount of heat that transfers through the shanks of mechanical fasteners is to minimize the quantity of fasteners that are utilized in joint areas. Unfortunately, a certain amount of fasteners are required in order to adequately transfer structural loads.
Furthermore, many aircraft typically include removable access panels or doors which allow for inspection and or access to certain internal portions of the aircraft structure. For example, fuel tanks in some aircraft may include removable panels that to allow for inspection of interior portions of the fuel tank. For such access panels, a plurality of mechanical fasteners are typically provided around the perimeter of the access panel. Unfortunately, heat is easily conducted through the shanks of such mechanical fasteners. The control of such heat flow to the fuel tank is of critical importance.
As can be seen, there exists a need in the art for a composite joint wherein the conduction of heat through the shank of the mechanical fasteners to the interior of the structure is minimized. Furthermore, there exists a need in the art for a composite joint that allows for effective sealing in the interior of the composite joint such that fluids may be effectively contained. In addition, there exists a need in the art for a composite joint that provides for effective sealing under dynamic structural loading of the composite joint such as may be caused by aerodynamic forces. Finally, there exists a need in the art for a composite joint that is of simple construction in order to simplify manufacturability, installation and maintenance.